A multiple optical fiber connector generally comprises a plurality of optical fibers which are aligned and adhesively fixed within the ferrule. The tips of the optical fibers protrude outwardly from the end face of the ferrule by predetermined lengths. The end faces of the ferrules of such multiple optical fiber connectors are faced toward each other so that the tips of their optical fibers are optically connected to each other.
The multiple optical fiber connectors include so-called MT (mechanically transferable) connectors, in which a pair of multi-fiber connector ferrules of pin fitting types are connected by a simple holder such as a clip, and MPO (multi-fiber push-on) connectors, which have housings with push-pull mechanisms and are coupled using a connector adaptor.
Such multiple optical fiber connectors may be manufactured as follows, for example. First, a plurality of optical fibers are aligned within a ferrule and fixed using an epoxy adhesive. The ferrule may be made of a polymer resin material (PPS resin, epoxy resin, etc.) containing a silica filler or a ceramic material (such as zirconia). Then, the epoxy adhesive covering the fibers is removed from the end face of the ferrule, and the end face of the ferrule is polished into a flat surface (flat surface polishing process). Subsequently, the end face of the ferrule is preferentially polished so that the fibers made of quartz-based glass protrude from the end face of the ferrule by a predetermined amount (protruding process). Then, after scratches and flaws are removed (removing process), the end face of the optical fiber is polished into a mirror face (finishing process).
The fiber end face subjected to a plurality of polishing steps as described above generally has a recess (core dip) in the core portion. The optical fiber consists of a core portion made of quartz glass doped with germanium (GeO2) or the like and a clad portion made of quartz glass. Since the core portion has a lower hardness than the clad portion, the core dip tends to become large through a plurality of polishing steps. In the finishing step for the end face of the quartz glass fiber, generally, a grinding stone or a polishing sheet containing cerium oxide (CeO2) abrasive grain is used to remove a work-affected layer and scratches. The core dip tends to become deeper and larger due to the mechanochemical action of CeO2.
The core dips of the fiber end faces affect connection loss, especially return loss in optical connection between multiple optical fiber connectors. The MM fiber may have a core diameter of 50 μm or 62.5 μm with respect to a clad diameter of about 125 μm. Since the core diameter is larger than that of the single mode fiber (SM fiber) (the core diameter is about 9 μm to the clad diameter of 125 μm), the core dip tends to become large and have significant influence.
Conventionally, an optical fiber having a core material consisting of a material harder than the clad material has been proposed for an MPO connector. This makes it possible, when polishing the optical fiber end face so that it protrudes from the ferrule end face for PC (physical contact) connection, to prevent formation of recesses in the cores, thereby preventing formation of a gap between the cores and improving coupling rate (JP-A-H10-82927: patent document 1).
Furthermore, a method of polishing connection end surfaces of multiple optical fiber connector ferrules (including end faces of optical fibers or protruding end faces) has been proposed for the multiple optical fiber connectors so that the optical fibers sufficiently protrude from the end face of ferrules while preventing formation of recesses in the fiber cores. The method comprises first, second and third polishing processes. In the first polishing process, adhesives adhered to the end surfaces are removed, and the end surfaces are made flat. In the second polishing process, the optical fibers are made to project relative to the connection end surface of the ferrule by constant amounts through polishing of the connection end surface using a satin polishing sheet. In the third polishing process, the projection dimension is changed to a predetermined projection dimension through polishing of protruding end faces of the optical fibers (JP-A-2002-18690: patent document 2).
A method for providing a plurality of optical fibers extending at least about 3.5 μm beyond the front face of the ferrule has been proposed, wherein the end portions of the optical fibers are substantially coplanar so as to establish direct physical contact between the optical fibers of multifiber connectors. In the method, the end portions of the optical fibers generally remain extended beyond the front face of the ferrule during the entire process, thereby obviating the need to grind or polish the end portions of the optical fibers flush with the front face of the ferrule prior to preferentially etching the front face of the ferrule relative to the end portions of the optical fibers. As a result of the relatively small size of the abrasive particles utilized, the end portions of the optical fibers may not have significant core dip (JP-A-2005-531032: patent document 3).